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not affected since they are reported with respect to the patient's head (the reference tool) and not
with respect to the Position Sensor.
If the Polaris Vicra cannot track the reference tool (for example, if the reference tool is occluded),
then the software will not be able to calculate the transformations of other tools with respect to the
reference tool.
3.10
Stray Marker Reporting
A stray marker is a marker that is not part of a rigid body. For example, by placing stray markers on
a patient's chest, the markers may be used to gate/track the patient's breathing in order to time
radiation therapy.
If the stray marker reporting functionality is enabled, the Polaris Vicra System will report tool
transformations, as well as 3D data (position only, no orientation information) for up to 50 markers
that are not used in tool transformations (including phantom markers, described below). It is then
necessary to eliminate phantom markers within the application software, and verify that the stray
markers are within the characterized measurement volume.
It is important to be aware of the potential hazards associated with using the stray marker reporting
functionality. The hazards are as follows:
•
An external IR source, for example, an IR transmitter or incandescent light, may be
identified as a stray marker. The position of a stray marker may be incorrectly identified and
return incorrect data.
•
No marker identification is possible from frame to frame. It is therefore the user's
responsibility to devise a method to keep track of which 3D position belongs to which
marker in which they are interested.
•
A stray marker does not have any angular position to know if marker normals are being
exceeded.
•
There are no built in checks to determine if the 3D result is a real marker or a phantom
marker, generated by other IR sources or markers in view of the Position Sensor. The system
tries to reject markers by the use of the line separation qualifier, but if several markers are in
a line parallel to the horizontal plane of the Position Sensor, phantom markers may still be
generated that are within the line separation qualifier.
•
Partial occlusion cannot be detected, or compensated for. The partial occlusion is not
detected by the Position Sensor, but the user may be able to detect the apparent shift if the
marker position can be constrained in the application software. For example, the marker
position has to be constrained along a vector and its position relative to another marker is
supposed to be fixed within some tolerance.
To enable the stray marker reporting functionality, use the 0x1000 reply option with the BX or TX
command. This reply option returns out-of-volume information along with the 3D data. See the
"Polaris Application Program Interface Guide" for details.
In order for the Position Sensor to measure stray markers, a tool definition file must be loaded, and
the associated port handle must be initialized and enabled, even if no tools are being tracked. The
Position Sensor illuminators emit IR light only when a tool definition file is loaded.
Polaris Vicra User Guide - Revision 2
How the Polaris Vicra System Works
37
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